Head to head
Which one will break you first?
Two fingerprints on one grid. Both are scored through your profile, so this answers the question for you rather than for the average player.
Nioh is the harder of the two for you, by 1.4 points. The gap is widest on execution, where Nioh asks considerably more than Age of Empires II: Definitive Edition. If you want the gentler run, start with Age of Empires II: Definitive Edition.
Axis by axis, biggest gap first
NIOH
Ki pulse turns stamina from something you spend into something you play in rhythm, and the game asks for it in the first hour without ever insisting you learn it.
The game is the antagonist. Good luck.
AGE OF EMPIRES II: DEFINITIVE EDITION
Build orders are the curve, and the campaigns teach almost none of what the game actually asks for. The hard AI does not forget its economy while it fights.
Sustained pressure. Progress is earned.